Preheat the oven to 250C with a baking tray inside.
For the dough: Mix all the ingredients by hand or with a mixer until the dough becomes smooth. Divide the dough into three portions and roll out very thinly on a lightly floured surface and then onto a baking sheet.
For the topping: Bring to room temperature half of the Saint Agur by breaking it down with a fork until you have a smooth texture. Mix the crème fraiche with the Saint Agur, the Dijon mustard and the honey. Season to taste with salt and pepper. Spread a third of that mixture onto the pastry dough and lay the sliced mushrooms on top of it.
Place the baking sheet with the pastry and the topping on the hot baking tray and bake for 6-8 minutes. When cooked, remove from oven and decorate with the ham, some rocket leaves and little cubes of Saint Agur leaving enough for the other two tartes. Repeat this process with twice or until you run out of pastry dough.